Measuring circuit



A Sept. 19,' 1944. .1. R. FORD MEASUBING czRcUIT Filed 001'.. 1, 1942 :inventor l Jahn Rhlrdv Patented Sept. 19, 1944 'MEAsUmNcf CIRCUIT vJohn R. Ford, Narberth, Pa., assignor to Radio Corporation of America, a corporation of Dela- Waffe Application October 1,1942, Serial No. 460,388 1o Claims.` (ci. 1v1-95) This invention relatesgenerally to electrical measuring circuits and particularly to a thermionic tube measuring instrument which includes means for minimizing the effect of variations of thermal tube currents upon the zero setting of the instrument over extended ranges.

Heretofore, vacuum tube voltmeters and simi-` lar instruments have had the inherent disadvantage of instability of zero setting. This condition has been due primarily to variations in anode current ldue to normal cathode temperature changes. Since means for controlling cathode temperature or normal plate .current are not always feasible in apparatus 'of this type, the instant invention contemplates the neutralization of such thermal currents as an effective means of obtaining desired stability.

The apparatus to be described is a typical thermionic tube voltmeter utilizing a diode rectier, and including`a voltage divider fromwhich selected potentials are applied to a conventional voltmeter. In order to neutralize eiectively the effect of thermal currents incident to variations in voltmeter range-or cathode temperature, a

second diode is utilized, and both diodes are con-` nected to a resistive network, in a bridge circuit wherein the thermal currents derived from the two diodes oppose and substantially neutralize each other.

Among the objects of the invention are to provide'a new and improved method of and means for measuring electrical quantities. Another object is to provide .a new and improved method rst diode I is connected to one terminal A of a iirst resistor 3. The remaining terminal E of the lrst resistor 3 is connected to one terminal of a second resistor 4. The remaining'terminal B of the second resistor 4' is connected to the cathode of the second diode 2. The resistors 3 and 4 should have substantially equal resistance.

The electrical midpoint E of the resistor network 3, 4 is connected to one terminal of a potentiometer l. The remainingvterminal of the potentiometer I is connected to ground. The

sliding contact 8 of the potentiometer 1 is connected, through Ia third resistor 9, to one terminal of and means for measuring electrical quantities wherein normal current variations within the measuring instrument are substantially neutralized to provide a highly accurate indication of the applied quantity. Still another object of the invention is to provide a new and improved method of and means for measuring electrical quantities which comprises a thermionic tube measuring instrument including means for compensating for normal cathode temperature changes and for thermal currents incident to variation of circuit-'elements as the instrument range is varied.-

The invention will bedescribed by referring to the accompanying drawing `o'i which the single gure is a schematic circuitdagram of one embodiment thereof. Y

Referring to the drawing, the voltmeter circuit includes aiirst diode thermionic tube I, thecathode of which is connected to ground, and a second diode thermionic tube 2, the anode of which is connected to ground. The anode of the of a conventional voltmeter II. The remaining terminal of the voltmeter II is connected to ground. If desired, a bypass capacitor I0 may -be connectedacross the voltmeter I I to eiectively segregate the A. C. and D. C. components of the voltages derived from the potentiometer 1.

. It should be understood that additional ampliflcation may be inserted in the circuit, or that triodes or other multielement thermionic tubes may be substituted, in any known manner, for the diodes described heretofore.

A first voltmeter input terminal C is connected through a rstv coupling capacitor 5 to ,the first terminal A of the iirst resistor 3. ,A second voltmeter input terminal D is connected through a second coupling capacitor 6 to the second/terminal B of the second resistor' 4. The iirst input terminal C is also connected to one stationary contact I2 of a single pole-double throw switch S2 and to the stationary contact I4 of a single pole-single throw switch SI. The second voltmeter input terminal D is connected to a second stationary contact I3 of4 the single pole-double throw switch S2 and to the movable contact I5 of the single pole-single throw switch A source of voltage e, to be measured, is connectedbetween the movable contact I6 of the y single pole-double throw switch lS2 and ground.-

Preferably, the rst and second diodes and 2 should be of similar construction and electrical characteristics. Normal thermal-current in the rst diode I will make the first terminalA of the rst resistor 3 negative with respect to ground. Normal thermal-currents in the second di'ode `2 will make\the second terminal B of the second resistor 4 positive withrespect .to ground. Since the resistance of the iirst resistor 3 is substantially equal to the resistance of the second resistor 4, andthe characteristics of the first diode I 'are'similar tothe characteristics of the second diode 2 thermal-currents (due to initial electron velocity of cathode emission) of the two voltage source e.

diodes will be equal in magnitude and effectively neutralizeeach other. Consequently, the electrical midpoint E of the` resistor network 3, 4 will be free of voltage variations due to variations in the diode characteristics. Likewise, since the electrical midpoint E of the resistor network is effectively at ground potential, insofar as thermal-currents are concerned, adjustment of the sliding contact B of the potentiometer 1 will not cause any change in the zero setting of the indicator Il. In practice, perfect neutralization of thermal-currents is not attainable, since the electrical characteristics of the diodes l and 2 cannot be made identical. However, the invention permits the reduction of the effects of ther-l mal-current variationsto a satisfactory value.

In operation, if the movable and stationary contacts l and I4, respectively, of the single polesingle throw switch SI are not in contact, and the movable` contact I6 of the single pole-double throw switch S2 is connected to the xed contact I3, the second diode 2 will operate as a rectifier providing a low impedance path for the negative peaks and an effective open circuit for the positive peaks of the' A. C. potentials applied thereto through the coupling capacitor 6. The positive voltage half cycles resulting at the point B, from the flow of current through the capacitor 6 during the time the negative half cycles are applied, will provide currents through the resistor 4 which will divide-first, through theresistor 3 and the nrst diode I to ground, and secondly, throughv the indicating circuit including theA voltage divider 'l to ground returning through the signal source e to the other side of the capacitor 6. Hence, indications proportional to the magnitude of the positive half cycles derived from the voltage source e will be obtained 'on the indicator l I. When the movable contact IB of the switch S2 is connected to the xed contact l2, the first diode l will operate as a rectier providing a low impedance path for the positive peaks and an eiective open circuit for the negative peaks of the A. C. potentials applied thereto through the coupling capacitor 5. The negative voltage half cycles resulting at the point A, from the flow of current through the capacitor 5 during the time the' positive half cycles are applied, will provide currents through the resistor 3 which will divide at the point E, and flow-rst, through the resistor 4 and the second diode 2 to ground; and secondly, through the indicating circuit including the voltage divider 1 to ground, returning through the signal source e to the other side. of the capacitor 5. Thus, indications proportional to the magnitudes of the negative half cycles derived from the A. C, voltage lsource e will be provided on the indicator Il.

When the single pole-single throw switchSl is closed, both diodes will operate as rectifiers, suchthat point A will acciunulate a negative voltage proportional to the positive peaks while point B will accumulate a positive voltage proportional to the negative peaks. The voltage at the electrical midpoint E will be equal to the difference of the accumulated voltages at points A and B and so will be proportional to the diierence between the magnitudes of the positive and negative voltage peaks derived from the A. C. source e. The voltage at the point E- is applied to the indicator circuit including the voltage divider 1, and the indicator Il will indicate the difference between the positive and negative peak values of the This indication will be unaffected by the contact position of the movable element i6 of the single pole-double-throw switch S2.

Thus the invention described comprises a simple and eiilcient means for neutralizing to a substantial degree variations in the zero setting of a vacuum tube ymeasuring instrument, as well as convenient means for measuring either, or both,-posi tive and negative peaks of an applied potential.

I claim as my invention:

1.'A thermionic bridge measuring circuit for a potential source includingiirst and second series connected resistors, a first thermionic tube, a second thermionic tube, means connecting the cath- 0de of said rst tube to the anode of said second tube, means connecting the anode of said first tube to the remaining terminal of said iirst resistor, means connecting the cathode of said second tube to the remaining terminal of said second resistor, means for applying said potential across at least one of said series resistors, a measuring instrument, and means connecting said instrument between the common terminal of said resistors and the common electrodes of said tubes.

2. A thermionic bridge measuring circuit for a potential source including iirst and second series connected resistors, a first thermionic tube, a second thermionic tube, means connecting the cathode of said first tube to the anode of said second tube, means connecting the anode of said first tube to the remaining terminalof said first resistor, means connecting the cathode of said second tube to the remaining terminals of said second resistor, means for applying vsaid potential to the bridge formed by said resistors and tubes, a measuring instrument, and means connecting said instrument between the commonterminal of said resistors and the common electrodes of said tubes.

3. A thermionic bridge measuring circuit for a potential source including iirst and second series connected resistors, a iirst thermionic tube, a sec-1 ond thermionic tube, means connecting the cathode of said rst tube to the anode of said second tube, means connecting the anode of said rst tube to the remaining terminal of said first resistor, means connecting the cathode of said sec- Aond tube to the remaining terminal of said second resistor, means for applying said potential to one of said series resistors, a measuring instrument. and means connecting said instrument between the common terminal of said resistors and the -common electrodes of said tubes.

4. A thermionic measuring circuit for a potential source including a first diode, a second diode, means connnecting the cathode of said first diode to a point of reference potential, means connecting the anode of said second diode to said point, resistive means, means connecting said resistive means to the remaining anode and cathode electrodes of said respective diodes, means `connecting said potential source to said point of reference potential and to one of said remaining diode electrodes, a measuring instrument, and means connecting said instrument to said reference potential point and to said resistive means,

5. A thermionic measuring circuit for a potential source including a first diode, a second 4diode, means connecting the cathode of said iirst diode ,to a point of reference potential, means 'connecting the anode of said second diode to said point, resistive means, means connecting said 'resistive means to the remaining anode and cathode electrodes of said respectiveAdiodes, means connecting said potential source -between said remaining diode electrodes, a measuring instrument, and means connecting said instrument to sistive means. l

6. A thermionic measuring circuit for a potential source including a first diode, a secondv diode, means connecting the cathode of said rst diode to a point oi reference potential, means connecting the anode `of said second diode to said' point, resistive means, means 'connecting said resistive means to the remaining anode and cathode electrodes of said respective diodes, reactive means connecting said potential source to said point of reference potential and to one of said remaining diode electrodes, a measuring instru' ment, and means connecting said instrument to said'reference potential point and to said resistive means. A

7. A thermionic measuring circ-uit for a poten- A 2,358,391 l said'reference potential point and to said reode electrodes tial source including a first diode, a second di'- l ode, means connecting the cathode of said iirst ,diode to a point of reference potential, means connecting the anode o! said second diode to said point, resistive means. lmeans 'connecting said resistive means to the remaining anode and cathode electrodesl of saidy respective diodes. reactive means connecting said potential source between said remaining diode electrodes, a measuring instrument. and means connecting'said intubes substantially to eiect neutralization of -thermal current variations -ln said tubes, a measstrument to said reference potential point and to said resistivemeans.

8. A thermionic measuring circuit for a poten-I tial source including a first diode, a second diode, means connecting the cathode of said rst diode to a point of reference potential, means connecting the anode of said second diode to said point, resistive means, means connecting vsaid resistive means to the remaining anode and cathode-electrodes of said respective diodes, reactive means connecting said potential source to said point of reference potential and to one of said remainingdiode electrodes, a measuring instrument, and means including a voltage divider connecting said instrument to said reference potential point and to said resistive means.

9. A thermionic measuring lcircuit for` a potential source including a rst diode, a second diode,` means connecting the cathode of said rst diode -to a point of reference potential, means connecting the anode of said second diode to saidV point, resistive means, means connecting said resistive means to thepremaining anode and cath of said respective diodes, reactive means connecting said source lbetween said remainlng diode electrodes, a measuring instrument, and means including a voltage divider connecting said instrument to said point of. refer'- ence potential a'nd to'said resistive means.-

10. A thermionic measuring circuit for a source of potential lincluding a nrst thermionic tube. a second thermionic tube, means connecting said uring instrument, means for selecting the polarity of said potential,V `means forapplymg said "selected potential to said circuit, and means including said instrument connected to said neutralization means to-indicate said potential.

JOHN R. man v 

